# Adapters

AMS does this through two components: **serializers** and **adapters**.
Serializers describe _which_ attributes and relationships should be serialized.
Adapters describe _how_ attributes and relationships should be serialized.
You can use one of the built-in adapters (```FlattenJSON``` is the default one) or create one by yourself, but you won't need to implement an adapter unless you wish to use a new format or media type with AMS.

## Built in Adapters

### FlattenJSON - Default

It's the default adapter, it generates a json response without a root key.
Doesn't follow any specifc convention.

### JSON

It also generates a json response but always with a root key. The root key **can't be overridden**, and will be automatically defined accordingly to the objects being serialized.
Doesn't follow any specifc convention.

### JSONAPI

This adapter follows **version 1.0** of the format specified in
[jsonapi.org/format](http://jsonapi.org/format). It will include the associated
resources in the `"included"` member when the resource names are included in the
`include` option.

```ruby
render @posts, include: ['authors', 'comments']
```

or

```ruby
render @posts, include: 'authors,comments'
```

The format of the `include` option can be either a String composed of a comma-separated list of [relationship paths](http://jsonapi.org/format/#fetching-includes), an Array of Symbols and Hashes, or a mix of both.

## Choosing an adapter

If you want to use a specify a default adapter, such as JsonApi, you can change this in an initializer:

```ruby
ActiveModel::Serializer.config.adapter = ActiveModel::Serializer::Adapter::JsonApi
```

or

```ruby
ActiveModel::Serializer.config.adapter = :json_api
```

If you want to have a root key for each resource in your responses, you should use the Json or
JsonApi adapters instead of the default FlattenJson:

```ruby
ActiveModel::Serializer.config.adapter = :json
```

## Advanced adapter configuration

### Registering an adapter

The default adapter can be configured, as above, to use any class given to it.

An adapter may also be specified, e.g. when rendering, as a class or as a symbol.
If a symbol, then the adapter must be, e.g. `:great_example`,
`ActiveModel::Serializer::Adapter::GreatExample`, or registered.

There are two ways to register an adapter:

1) The simplest, is to subclass `ActiveModel::Serializer::Adapter`, e.g. the below will
register the `Example::UsefulAdapter` as `:useful_adapter`.

```ruby
module Example
  class UsefulAdapter < ActiveModel::Serializer::Adapter
  end
end
```

You'll notice that the name it registers is the class name underscored, not the full namespace.

Under the covers, when the `ActiveModel::Serializer::Adapter` is subclassed, it registers
the subclass as `register(:useful_adapter, Example::UsefulAdapter)`

2) Any class can be registered as an adapter by calling `register` directly on the
`ActiveModel::Serializer::Adapter` class. e.g., the below registers `MyAdapter` as
`:special_adapter`.

```ruby
class MyAdapter; end
ActiveModel::Serializer::Adapter.register(:special_adapter, MyAdapter)
```

### Looking up an adapter

| Method | Return value |
| :------------ |:---------------|
| `ActiveModel::Serializer::Adapter.adapter_map` | A Hash of all known adapters `{ adapter_name => adapter_class }` |
| `ActiveModel::Serializer::Adapter.adapters` | A (sorted) Array of all known `adapter_names` |
| `ActiveModel::Serializer::Adapter.lookup(name_or_klass)` |  The `adapter_class`, else raises an `ActiveModel::Serializer::Adapter::UnknownAdapter` error |
| `ActiveModel::Serializer::Adapter.adapter_class(adapter)` | Delegates to `ActiveModel::Serializer::Adapter.lookup(adapter)` |
| `ActiveModel::Serializer.adapter` | A convenience method for `ActiveModel::Serializer::Adapter.lookup(config.adapter)` |

The registered adapter name is always a String, but may be looked up as a Symbol or String.
Helpfully, the Symbol or String is underscored, so that `get(:my_adapter)` and `get("MyAdapter")`
may both be used.

For more information, see [the Adapter class on GitHub](https://github.com/rails-api/active_model_serializers/blob/master/lib/active_model/serializer/adapter.rb)